1. Field of the Invention
The present invention relates generally to locator systems and techniques, and more particularly, to finding systems for determining the location of objects and/or subjects.
2. Description of the Related Art
Most systems for locating a subject or object employ the use of direction locating antennas to determine the position of the subject. However, such locating systems are characterized by shortcomings associated with the size of the antenna at the bandwidth that is optimal for the application. Direction locating antennas experience significant degradation of directional capabilities in close range conditions wherein the separation between a search unit and a target is about several hundred feet or less.
It is well known that there is a correlation between antenna size and RF wavelength. A larger antenna is needed for a longer RF wavelength. The need for small antenna size forces the selection of relatively high frequency bands of 900 MHz and higher where there is a lot of interference in the form of reflections and where there is considerable signal degradation as the signal passes over small objects or obstacles. In short, relatively high frequency bands are not suited for searches where the separation between the search unit and the target is greater than a hundred feet.
Moreover, the use of directional antennas precludes coordinated searches wherein several search units are homing in on a target or are tracking multiple targets. The use of directional antennas also precludes monitoring a plurality of subjects at the same time because a monitoring unit employing a directional antenna cannot receive and transmit signals in multiple directions. Because of significant directional errors that are associated with directional antennas, the operator also is required to have special skills in performing the search, i.e., locator systems employing directional antennas are not user friendly.
Known locator systems rely on distance measurement to determine the separation between a monitoring unit and a subject whose location is being monitored. Distance measurement generally is carried out either by measuring signal strength or by measuring the propagation time between sending a ranging signal and receiving a ranging signal. Examples of systems that use signal strength to determine distance to locate a subject are disclosed in U.S. Pat. No. 5,086,290 and in U.S. Pat. No. 5,650,769, for example. Systems that rely on measurement of signal strength are prone to be unreliable due to noise, interference, signal strength changes, reflections, etc. as well as signal degradation as the signals pass over obstacles. Moreover, measurement error is a function of signal strength, whereby large signal attenuation typically occurs within a building as opposed to outside of a building. In these systems accuracy of measurement is distance dependent, whereby if the distance change is small such systems function appropriately, although, they are known to be less accurate at larger distances.
Another system disclosed in U.S. Pat. No. 5,525,967 uses timing to determine distance. Time measurement does not rely on signal strength and is immune to the signal attenuation. Also, the distance measurement error is constant and does not distant depend signal attenuation. Some of the known time measurement locator systems rely on variations of directional antennas, for example a phase array antenna. Such variations allow the reduction of antenna size. However, the price for these improvements is a complex antenna design and an extremely complex signal processing requirements, which result in a lower accuracy, higher cost and power consumption. Also, such antennas are subject to operating frequency limitations and require a wide bandwidth. Known distance measurement systems that employ time-measurement techniques require a large bandwidth in order to achieve a desired accuracy. This results in increased interference, higher circuit complexity and power consumption as well as higher cost. Wide bandwidth requirements also limit the number of devices that can operate simultaneously within a given band. These devices have wide bandwidth requirements that have particular disadvantages such as, for example, such devices cannot operate on business or otherwise unlicensed bands that prohibit ease of the units to transmit and receive in an unregulated environment, limit the units from being sold “over the counter” or integrated with mass-produced popular hand-held radios.
In United States Patent Publication No. 2002/0155845, a position location system is disclosed that uses spread spectrum technology for determining range information in a severe multi-path environment. The system uses ranging processes wherein ranging pulses at eight different frequencies within a band are exchanged between a master radio unit and each of at least four reference radio units. The position and velocity information obtained by the ranging process enables determination of the position of the master radio's position in three dimensions. This system uses a variation of time-measurement based techniques for distance determination. As a result, it carries all of the drawbacks mentioned above plus its operation frequencies and or bands are limited. The system does not employ a directional antenna. Instead, it uses additional four fixed references with known coordinates, or four mobile references that have their coordinates continuously updated via GPS or manually. This system allows simultaneous operation of many units. In this system, the usage of a directional antenna is eliminated. However, the system has disadvantages that include adding a complex infrastructure requiring multiple references, fixed and or mobile, that all include GPS or otherwise need continuous manual updating of coordinate data; limited operating band; increased complexity of the system both technological and logistical; cost; and power consumption. As a result, the system has a very narrow usage in specialized applications. The present invention overcomes such disadvantages of the prior art to provide methods and devices for finding subjects and objects that reduce and or eliminate the infrastructure overhead, for example, the present invention operates without (i) usage of a directional antenna, (ii) any position references, and or (iii) operating band limitations so as to lower the complexity of the system and the overall cost of the devices.